Implementation of new European directives in the environmental field and, specially, in the water field, generate a request from policy-makers for news tools able to evaluate impact of measures token in order to reduce pressures on ecosystems. In this context, the objective of this PhD thesis is to develop a “soil and vadose” hydrological model in order to evaluate impact of diffuses pollutions and mitigation measures on water quality in the Walloon region. The “soil and vadose zone” model of the Walloon region (EPICgrid model) has been developed at catchment scale with an original modular concept on the basis of the field scale “water-soil-plant” EPIC model (Williams et al., 1984). After that the model was validated at field scale and at catchment scale, the EPICgrid model was used to simulate the hydrological behaviour of the Walloon region catchments and to produce environmental indicators for surface water and groundwater bodies of the Walloon region. The simulated fluxes are related to unsaturated or temporarily saturated soils. The simulations are focused, on the one hand, on hydrological balance terms (direct runoff, subsurface fluxes, groundwater recharge, soil moisture variability, actual evapotranspiration) and, on the other hand, on associated fluxes of nitrogen, phosphorus, organic matters and sediments, from agricultural origin. The EPICgrid model was then used to evaluate water quality impact of mitigation measures token in order to reduce agricultural diffuse pollution effect on the quality on surface water and groundwater bodies in the Walloon region.